leds: uleds: Fix potential buffer overread

In the Linux kernel, the following vulnerability has been resolved: leds: uleds: Fix potential buffer overread The name string supplied by userspace is not guaranteed to be null-terminated, so using strchr() on it might result in a buffer overread. The same thing will happen when said string is used by the LED class device. Fix this by using strnchr() instead and explicitly check that the name string is properly null-terminated.